Compatibility layer

In software engineering, a compatibility layer is an interface that allows binaries for a legacy or foreign system to run on a host system. This translates system calls for the foreign system into native system calls for the host system. With some libraries for the foreign system, this will often be sufficient to run foreign binaries on the host system. A hardware compatibility layer consists of tools that allow hardware emulation.

FreeBSD's Linux compatibility layer, which enables binaries built specifically for Linux to run on FreeBSD[2] the same way as the native FreeBSD API layer.[3] FreeBSD also has some Unix-like system emulations, including NDIS, NetBSD, PECoff, SVR4, and different CPU versions of FreeBSD.[4]

The PEACE Project (aka COMPAT_PECOFF) has Win32 compatible layer for NetBSD. The project is now inactive.

A compatibility layer avoids both the complexity and the speed penalty of full hardware emulation. Some programs may even run faster than the original, e.g. some Linux applications running on FreeBSD's Linux compatibility layer may perform better than the same applications on Red Hat Linux. Benchmarks are occasionally run on Wine to compare it to Windows NT-based operating systems.[7]

Even on similar systems, the details of implementing a compatibility layer can be quite intricate and troublesome; a good example is the IRIX binary compatibility layer in the MIPS architecture version of NetBSD.[8]

A compatibility layer requires the host system's CPU to be (upwardly) compatible to that of the foreign system. For example, a Microsoft Windows compatibility layer is not possible on PowerPC hardware because Windows requires an x86 CPU. In this case full emulation is needed.